1. Field of the Invention
The present invention relates to a method as well as an installation for purifying contaminated water coming from a carwash.
2. Description of the Prior Art
Due to stricter environmental legislation it is in some cases no longer allowed to discharge the contaminated water coming from a carwash directly into the environment. It is now compulsory for such companies to purify the contaminated water before it may be discharged. It is preferred however that the degree of purification is such that it is possible to re-use the purified water in the carwash. Re-use of purified water is further preferred because of a shortage of clean water and because water is becoming more and more expensive.
Many techniques are known for purifying contaminated water. The technique applied in practice depends on many factors, such as the nature of the contamination in the water, the desired degree of purification, cost and the like. An example of a purification technique is the use of sand filters. The most important function of the sand filter is the filtering out of the contaminants. During filtration other processes can however also occur, such as adsorption of substances to the sand particles and degradation of organic material by micro-organisms.
The currently known purification techniques do not always provide the desired result. With many tested techniques the desired degree of purification was not obtained or the operational management was uncertain. Other techniques which did provide the desired result were found to be unfeasible from the aspect of cost.
Operational reliability is moreover essential, because the purified water is preferably used again in the cleaning proces of the car wash. A purification installation which does not operate or operates badly can for example result in damaged cars, when the re-used water still contains relatively coarse particles.
It is therefore the object of the present invention to provide a method and an installation for purifying contaminated water coming from a car wash, wherein the desired degree of purification is obtained, the operational management is certain and both the investment costs and the operational management costs are more favorable than with application of currently known techniques.
This is achieved by the invention by a method comprising the steps of:
pretreating the contaminated water such that the size of the organic material it contains is smaller than a predetermined cut off value;
subsequently carrying the contaminated water in upward direction through a packed bed of particulate material, in which micro-organisms produce exo-enzymes to degrade the organic material in the contaminated water, which bed is aerated from below, and draining the purified water from the packed bed at a level located a determined distance from the top of the bed; wherein
an additive is added to the contaminated water to stimulate production of exo-enzymes by the micro-organisms in the packed bed.
Prior to feeding of the contaminated water to the packed bed, the contaminated water is pretreated such that the organic material it contains is smaller than a predetermined cut off value. Because of this the functioning of the packed bed can be optimized. The cut off value depends mainly on the desired degree of purification. Experimentally it is found that a cut off value of 100 xcexcm provides a satisfactorily purification degree. However, when a cut off value of 20 xcexcm is taken into account it is possible to re-use the purified water for high pressure car wash installations. Most preferably the cut off value is 5 xcexcm such that a optimal functioning of the packed bed is obtained. In this case sludge forming and accumulation of bio mass at the top of the bed is prevented, whereby the period of operation of the bed is longer.
In the upward flow of water through a packed bed of particulate material the problem of expansion could occur. The bed expands due to the flow, the packing is disturbed, xe2x80x9ccratersxe2x80x9d are created and adequate filtration can no longer take place. Therefore the water is drained at a level located a determined distance from the top of the bed before the water reaches the uppermost part of the bed. The top layer of the bed ensures that the particulate material of the bed remains well packed, such that expansion is prevented and the filter action preserved.
The bed is aerated from below to provide sufficient oxygen for the micro-organisms to produce exo-enzymes which degrade the organic material in the contaminated water. The aeration preferably takes place by means of block aerators consisting of a relatively flat housing provided on at least one side with a plate of porous material, e.g. sintered plastic. The housing comprises an inlet opening for air, which air flows out again as small bubbles via the porous plate. The use of porous sintered plastic, in particular polyethylene, results in an exceptionally good distribution of the supplied air. In addition, the material is very inexpensive compared with alternative materials and it has been found that the material is not susceptible to contamination and is thereby operationally very reliable. However, other aerating means can of course also be applied such as tubes with apertures.
Further an additive is added to the contaminated water to stimulate production of exo-enzymes by the micro-organisms in the packed bed. These exo-enzymes degrade the organic material in the contaminated water. The additive is preferably added immediately before the contaminated material is carried through the packed bed, such that no additive is lost in the pretreating step during purification of the contaminated water. Furthermore, the additive preferably consists of at least one of a vegetable extract and a nutrient medium for providing extra nutrition for the micro-organisms and ensuring the micro-organism will keep growing even when the purification installation is not running. Preferably the micro-organisms are bacteria. It is found that micro-organisms, and especially bacteria, are functioning extremely well when the particulate material of the packed bed consists of bio carriers. These are canister like elements made of plastic, for example polyethylene, with a open structure. The open structure provides an enlarged area on which the bacteria can settle and ensures a good flow through the carriers.
According to one aspect of the present invention the step of pretreating comprises the step of reducing the size of the organic material in the contaminated water. This is preferably effected by means of grinding. Contaminants, especially particles which will not settle and remain floating in the contaminated water, e.g. cigarette butts and bristles, will be reduced in size.
According to another aspect of the present invention the step of pretreating comprises the step of removing from the contaminated water organic material with a size larger than the cut off value. This is preferably effected by means of a hydrocyclone. The contaminated water leaving the hydrocyclone as the upperflow contains contaminants of a size smaller than the cut off value, whereas the underflow of the hydrocyclone besides some contaminated water contains contaminants of a size larger than the cut off size. It is possible, and preferable, to let the underflow flow back to the beginning of the pretreating step.
The step of pretreating can, according to the invention, in addition comprises the step of removing volatile components from the contaminated water by means of stripping. Volatile components, in particular hydrocarbons, for example gasoline and light fractions of oil, can thus be removed from the contaminated water and possibly be treated before it is brought into the environment. Preferably the stripping is effected by means of blowing air through the contaminated water. Next to removal of volatile components from the contaminated water, the growth of anaerobic bacteria is impossible.
Contaminants which are not separated out in the pretreating step and which are not degraded in the filter bed, mainly inert material such as sand, will eventually leave the bed together with drainage of the purified water. The purified water is collected in a clear water container, in which the inert material not being removed will settle. These settlings are removed from the clear water container by means of bottom overflow and can flow back to the beginning of the purification proces. Purified water can be withdrawn from this clear water container by means of a pump.
The present invention further relates to an installation for performing the method according to the invention. The installation comprises means for pretreating the contaminated water such that the size of the organic material it contains is smaller than a predetermined cut off value, said pretreating means being connected to means for transporting the pretreated contaminated water to a packed bed of particulate material, in which micro-organisms produce exo-enzymes to degrade the organic material in the contaminated material, said packed bed comprising a duct for introducing the contaminated water near the bottom of the bed, a duct for draining purified water from the bed at a level located a determined distance from the top of the bed and an aerator located near the bottom of the bed, wherein the installation further comprises an inlet point for adding an additive to the contaminated water to stimulate production of exo-enzymes by the micro-organisms in the packed bed. Further aspects and advantages of the installation are described below with reference to the accompanying drawings, in which a preferred embodiment of the invention is depicted.